1. Field of the Invention
This invention relates generally to high intensity discharge electrodeless lamps and more particularly to an improved apparatus for starting an electrodeless lamp without the need for an external starting aid.
2. Description of Related Art
Electrodeless lamps comprise light sources which are based on generating light directly by a plasma generated and sustained by RF excitation. This is opposed to fluorescent lamps where electrical excitation of a plasma, which may include mercury, produces ultraviolet photons which in turn impact on phosphors located on an inside surface of a tubular body which produces light as a secondary product.
As is generally well known, an electrodeless lamp is typically comprised of a closed, transparent quartz container having a low pressure fill consisting of an inert gas and selected chemical elements. Typically, the gas is comprised of argon and element(s) selected from Group VI-A of the periodic table of elements, e.g. sulfur. The material in the bulb is heated by the concentration of RF energy applied to the bulb, whereupon the inert gas is ionized and the supplementing elements are vaporized to form a high temperature plasma which emits highly concentrated light. Where sulfur is used as the element being vaporized, the light which is generated is similar to bright sun light.
RF coupling to the bulb can be either inductive, capacitive or by way of a microwave cavity. Where, for example, coupling to the bulb is inductive through an excitation coil wound about the bulb, and RF power is applied to the bulb in the order of 100-200 watts at a frequency of 10 MHz-100 MHz, typically 27.12 MHz, which is in the industrial, scientific, medical (ISM) band, difficulty is often encountered in starting the plasma generation process. The use of a Tesla coil, an external high voltage field producing device, or other forms of starting aids are required to induce ignition.